The invention relates to superconducting ceramic compositions. More particularly, the invention relates to superconducting ceramic materials of the kind termed 1212 with a unit cell containing two perovskite structure copper-oxygen planes (within which the supercurrent is confined) which are positioned between non-superconducting rocksalt structure layers comprising single planes of metal and oxygen ions (which supply the charge carriers necessary for the supercurrent).
The strong coupling of the charge carrier wavefunctions across the single planes of metal and oxygen ions means that superconducting ceramic materials with the 1212 structure exhibit stronger pinning of the magnetic flux lines in liquid nitrogen than materials with thicker rocksalt structure layers containing two or more planes of metal and oxygen ions.
A series of layered superconducting materials with 1212 structure are already known in which the cation composition of the insulating rocksalt structure layer is (Tl.sub.1-x Bi.sub.x), (Tl.sub.1-x Pb.sub.x) or (Pb.sub.1-x Cu.sub.x). Examples of known compositions are: EQU (Tl.sub.0.5 Bi.sub.0.5) Sr.sub.2 CaCu.sub.2 O.sub.7, EQU (Tl.sub.0.5 Pb.sub.0.5) Sr.sub.2 (Y.sub.0.2 Ca.sub.0.8) Cu.sub.2 O.sub.7, EQU (Pb.sub.0.5 Cu.sub.0.5) Sr.sub.2 (Y.sub.0.5 Ca.sub.0.5) Cu.sub.2 O.sub.7,
The prior art ceramic oxides pose the following problems:
(1) Although the thallium-containing ceramics exhibit the highest critical temperatures, the toxicity of thallium is a major problem in their large-scale utilisation.
(2) Due to thallium's high reactivity, thallium-containing ceramics are subject to degradation. A more chemically-stable substitute which maintains the desired superconducting properties has been the subject of intense research.
(3) With a lead-copper mixture in the rocksalt structure layer, the oxygen stoichiometry is difficult to control with the result that the optimum superconducting properties are not achieved. Examples of such elaborate techniques employed to achieve the optimum critical temperature in these materials are the use of high-pressure oxygen annealing in specialist furnaces or the quenching of samples f rom temperatures above 800.degree. C. in air. Such procedures are not viable for industrial production.